1. Field of the invention
The present invention relates to aluminium borate whiskers and a preform thereof, and more particularly to a method of manufacturing aluminium borate whiskers and a preform thereof having a reformed surface based upon a gamma alumina layer generated during the manufacture of the aluminium borate whiskers.
2. Description of the Prior Art
Aluminium borate whiskers have a composition generally expressed by Al.sub.18 B.sub.4 O.sub.33 and are superior in strength, elasticity, heat resistibility and anti-chemical resistance, thereby providing a superior reinforcing material for a composite material. Nevertheless, aluminium borate whiskers are available at very low cost, so that an aluminium base composite material reinforced by aluminium borate whiskers is available at much lower cost than a composite material reinforced by, for example, SiC whiskers.
The aluminium borate whiskers generally have a uniform r-alumina (gamma alumina) layer covering the surface thereof just as they have been manufactured, and the r-alumina layer is removed before the whiskers are delivered as a product, as the r-almuina is thermally and chemically unstable. In this regard, it was proposed in Japanese Patent Laid-open Publication 5-139899 to heat the aluminium borate whiskers bearing a uniform r-alumina layer remained thereon so that the r-alumina layer is converted into a layer of .alpha.-alumina (alpha alumina) which is stable at high temperature.
However, in a composite material having an aluminium alloy matrix reinforced by the aluminium borate whiskers, a reaction EQU Al.sub.18 B.sub.4 O.sub.33 +4Al=11r-Al.sub.2 O.sub.3 +4B
occurs, and since B is less reactive with oxygen than Mg, there further occurs a reaction EQU r-Al.sub.2 O.sub.3 +Mg+[0]=MgAl.sub.2 O.sub.4
whereby a layer of spinel (MgAl.sub.2 O.sub.4) is generated along the surface of the whiskers, thereby decreasing the amount of Mg in the matrix, while the strength of the whiskers lowers, deteriorating the strength of the composite material.
Various attempts have been made to obviate the above-mentioned problem caused by the generation of the spinel, including an improvement of the composition of the matrix metal or a reforming of the surface layer of the whiskers.
However, it was noted that, even when the aluminium borate whiskers have been treated according to the method of the abovementioned publication, when the aluminium borate whiskers are heated to 800.degree.-1200.degree. C. in the atmosphere during a manufacture of a composite material in which an aluminium alloy containing Mg is reinforced by the aluminium borate whiskers, the spinel is still generated along the surface of the whiskers, showing that the effect of suppressing the generation of the spinel is not yet sufficient.